Although iron is the most abundant transition metal in the earth's curst it is highly inaccessible under ambient conditions because of the insolubility of Fe(OH)3. In response to the challenge posed by iron requirement and unavailability, microorganisms secrete high-affinity iron-binding compounds called siderophores with several types of chemical structures found among the more than 100 compounds characterized to data. Siderophores solubilize ferric ion and enable it to be transported into cells, usually via specific membrane-bound uptake systems. This process involves a highly stereospecific recognition of the ferric complex by receptor proteins at the cell surface. Examination of the solution conformations and geometries of these iron-complexing agents is made possibly by the preparation of exchange inert metal complexes. Fundamental data on solution thermodynamics, kinetics, and electrochemistry are determined to examine iron release mechanisms. The application of Mossbauer spectroscopy in vivo allows direct monitoring of iron assimilation via siderophores. Specific goals are: to determine the stereochemical specificity of siderophore recognition and transport using exchange inert metal complexes and synthetic siderophore analogs; to categorize siderophore-mediated iron transport; to characterize new siderophores, their structures, physical properties and transport behavior; to examine the fundamental thermodynamic features of siderophores; to examine relative rates of metal complexation and exchange for siderophores and transferrins to determine whether such removal is a pathway for iron uptake by potential pathogens; to understand the electronic structure of these high-spin d5 systems so that the VIS/UV and CD spectra of the native siderophore iron complexes can be used more effectively in structure and composition assignment. Iron availability is a key factor in the virulence of microbial infections. Siderophores have been identified as germination or sporulation factors for several organisms and human pathogens obtain iron from tissues or sera by producing siderophores. Diseases in which direct involvement of siderophores has been established include infantile enteritis, leprosy, and tuberculosis. Iron transport is now known to be essential in the growth of many food plants (and their pathogens) as well as in the growth of phytoplankton (the base of the marine food chain). The focus of this research project remains the coordination chemistry aspects of siderophore mediated iron transport in bacteria and fungi.